To establish a communication link between two wireless devices, a wireless device receiving a signal transmitted by the other wireless device must synchronize receiver processing with the received signal. Both time synchronization and frequency synchronization are required, to achieve optimal demodulation and decoding of information carried by the received signal. This time synchronization and frequency synchronization may then be applied to signals transmitted by the wireless device. For example, time synchronization and frequency synchronization are necessary for a wireless device (often referred to as a user equipment, or “UE,” in industry terminology) to access a wireless network, whether as part of a random access (RA) process for initial access, or for a transition by the wireless device from an idle state to an active state.
To aid the synchronization process, standardized air interfaces typically provide for the transmission of synchronization, or “synch,” signals. One example is the synchronization signals transmitted in the downlink (base-station-to-user-equipment transmissions) in the Long-Term Evolution (LTE) networks standardized by members of the 3rd-Generation Partnership Project (3GPP). These downlink synchronization signals in LTE comprise two components: a primary synchronization signal (PSS), used for coarse frequency synchronization and symbol time estimation signal, and a secondary synchronization signal (SSS), used for frame timing estimation. For convenience, cell-identity information is also encoded into the PSS and SSS signal, in LTE systems.
In LTE, the synchronization signals are transmitted twice in each 10-millisecond radio frame. In Frequency-Division Duplexing (FDD) mode, the PSS is transmitted in the last Orthogonal-Frequency Division Multiplexing (OFDM) symbol of the first and eleventh slot of each radio frame. This permits the UE to acquire slot-boundary timing, without having to worry yet about the cyclic prefix length. (Time-Division Duplexing, or TDD, mode uses a different frame structure that is not detailed here but that is described, along with the FDD-mode structure, in 3GPP TS 36.211 v8.5.0, “Physical Channel and Modulation,” December 2008.) The PSS uses a sequence known as Zadoff-Chu.
More specifically, in the frequency domain, the PSS occupies the central six resource blocks (RBs) of the LTE downlink signal, no matter what the allocated bandwidth of the signal is. This allows the UE to synchronize to the network without knowing the allocated signal bandwidth. (The SSS also occupies these same six resource blocks). The synchronization sequence, i.e., the Zadoff-Chu sequence in the case of the PSS, is 62 symbols long, and is mapped to 62 OFDM subcarriers, with 31 subcarriers mapped on each side of the DC sub-carrier, which is not used. Because each RB includes 12 subcarriers, this leaves five subcarriers at the ends of this six-RB group unused.
The SSS in the FDD-mode LTE downlink signal is transmitted in the next-to-last symbol of the first and 11th slot of each radio frame, and is thus immediately before the PSS. This makes it easy for the UE to determine the cyclic prefix length that is in use, by comparing the timing of the PSS and the SSS. (In LTE, there are two cyclic prefix lengths that are possible, and hence two different OFDM symbol lengths.) Each instance of the SSS signal, which is based on so-called M-sequences, is also a 62-symbol sequence mapped to same subcarriers as the PSS. However, the SSS signal alternates in a predetermined manner from one transmission to the next. This allows the UE to determine the position of the 10-millisecond frame boundary.
A key part of the synchronization process is timing offset estimation, which is typically performed by comparing the received signal to multiple reference signal hypotheses that correspond to different offsets in time. In LTE UEs, the bulk of the searching effort is associated with time-domain correlation of the PSS for different time-offset hypotheses. Out of the correlation results for many hypotheses, the largest correlation peak is used for determining the time-offset estimate.
As suggested above, a synchronization signal is typically based on a pre-defined sequence of symbols, referred to as a synchronization sequence. These synchronization sequences are carefully chosen to ease the synchronization process. A common type of synchronization sequence, which is used in the LTE PSS, is the Zadoff-Chu design. Due to the favorable cross- and auto-correlation properties of sequences that fall in this family of sequences, each synchronization sequence from the Zadoff-Chu family provides good robustness with respect to other sequences from the same family, as well as with respect to time-shifted copies of itself. Thus, correlation peaks corresponding to incorrect time alignment hypotheses are few and relatively low, so false alarm probabilities can be kept low.
To coherently detect the classical synchronization sequences, including Zadoff-Chu synchronization sequences, the receiver must maintain substantial phase coherence over the entire length of the sequence, e.g., the OFDM symbol during which the PSS is transmitted. Therefore, in the presence of any non-negligible frequency uncertainty, the correlation reference sequences also need to incorporate multiple frequency-offset hypotheses, in addition to the time-offset hypotheses discussed above. For each frequency-offset hypothesis, the reference sequence is distorted in accordance to the particular frequency offset. Multiple time-shifted versions of this distorted reference sequence are compared to the received signal; the best correlation result from these multiple trials indicates the most likely time offset and frequency offset.
It should be appreciated that searching across multiple frequency hypotheses in addition to across multiple time offsets has a multiplicative effect on the number of hypotheses that must be tested, and thus increases the computational burden of the synchronization process. Techniques that reduce this computational burden are thus desired.